One common method used in the industry to produce tubular shapes is the longitudinally welded U-ing/O-ing process. It is a common occurrence that at the end of this process, due to the thermal stresses induced by the welding and cooling rate of the weld, the resulting tubular product is bowed, which is also commonly referred to in the industry as “banana shaped.”
Therefore, an additional operation is necessary to straighten the product within industry accepted tolerances. This process, as known from the state of the art, has numerous deficiencies.
A common practice in tube straightening is to first measure, which is often done manually, the bow of the tubular product, generally at multiple locations. Then, a straightening apparatus is set with physical stops, or hard stops, at the same locations where the measurements were taken, and on the opposite side of the bow.
The rule of thumb is that the hard stops are set to allow a deflection approximately two times the value of the measured bow. The tube is then bent until it touches the physical stops, and then relaxed, and the bow is measured again. If the result is not satisfactory by given acceptance criteria, the hard stops are adjusted to allow for more bending.
When a second bending is required, one problem with the above-described process is that the adjustment of the hard stops is entirely experience-based, and, therefore, there is an increased risk of denting the tube, which results in an unacceptable product. One other problem of the above-described process is that, the longer the tube, the more measurements need to be taken, and more hard stops have to be set.
The above-described conventional process is slow, labor intensive, requires highly skilled operators, and is subject to error. Further, the process is highly dependent on a ratio of the diameter to the wall thickness of the tube, i.e. (D/t). While this ratio is a constant value for straight tubes, in the case of taper tubes, this ratio varies along the length of the tube. No method is available, other than operator experience, to take this ratio into account, let alone to deal with a variable ratio.
Even more, the physical properties of the tube material are very important, and there is no way to take them into account with the described state of the art.